Cleavage of non-polar C(sp2)‒C(sp2) bonds in cycloparaphenylenes via electric field-catalyzed electrophilic aromatic substitution

Electrophilic aromatic substitution (EAS) is one of the most fundamental reactions in organic chemistry. Using an oriented external electric field (OEEF) instead of traditional reagents to tune the EAS reactivity can offer an environmentally friendly method to synthesize aromatic compounds and hold the promise of broadening its scope. Despite these advantages, OEEF catalysis of EAS is difficult to realize, due to the challenge of microscopically orienting OEEF along the direction of electron reorganizations. In this work, we demonstrate OEEF-catalyzed EAS reactions in a series of cycloparaphenylenes (CPPs) using the scanning tunneling microscope break junction (STM-BJ) technique. Crucially, the unique radial π-conjugation of CPPs enables a desired alignment for the OEEF to catalyze the EAS with Au STM tip (or substrate) acting as an electrophile. Under mild conditions, the OEEF-catalyzed EAS reactions can cleave the inherently inert C(sp2)-C(sp2) bond, leading to high-yield (~97%) formation of linear oligophenylenes terminated with covalent Au-C bonds. These results not only demonstrate the feasibility of OEEF catalysis of EAS, but also offer a way of exploring new mechanistic principles of classic organic reactions aided by OEEF.


Molecular junction optimization and transmission calculations
We attach two Au clusters containing 4 Au atoms (Au-Au bond length is constrained to 2.88 Å) to the two sides of the optimized molecular structure and relax the junction geometries using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional implemented by the Fritz Haber Institute ab initio molecular simulation (FHI-aims) packages. After relaxation, the 4 atom Au clusters are replaced by Au pyramids containing 60 Au atoms in 6 layers. The Landauer transmission across these junctions is calculated using the nonequilibrium Green's function (NEGF) formalism.

Molecular junction optimization under electric field
We first attach one Au cluster containing 2 Au atoms to the optimized [6]CPP molecule to form an Au-CPP π-complex and relax its geometry using the PBE exchange-correlation functional implemented by the FHI-aims packages, all-electron numeric atom-centered basis set (light computational settings) is used. After geometry optimization, we apply an external homogeneous electric field with orientation aligned along the direction of Au-π bond. We relax the Au-CPP complex geometries under electric field of different strength between 0.05 to 0.5 V/Å.

Control STM-BJ measurements in pure 1-chloronaphthalene solvent
We have performed control STM-BJ measurements in pure 1-chloronaphthalene solvent, and did not observe any obvious molecular conductance features (see Figure   R3). This confirms that the observed single molecule junction signatures arise from the CPP molecules instead of the solvent.

Data clustering for [6]CPP
To gain more quantitative information of the high bias induced transition from the High-G junction to the Low-G junction, we sort the conductance traces measured at each bias into three categories, corresponding to the High-G junction (II), the Low-G junction (III) and the direct tunneling nanogap (IV), see Figure S2. We note that the probability of junction formation is around ~50% for all the measurements. Moreover, the Low-G junction yield (YLow-G), determined by the ratio of the number of the Low-G junction traces to the total numbers of junction traces, increases with the increase of the applied tip bias. More specifically, at the low bias of 0.1 V, the YLow-G is as low as 6.25%. In sharp contrast, the YLow-G increases by ~15-fold and reaches 97.32% at the high bias of 1 V. These results indicate that the increase of the high bias to 1 V leads to an almost complete transition from the High-G junction to the new type of Low-G junction.   Table S1 below. Fig. 10. Conductance versus number of phenylene units. The conductance data for the covalent Au-C bonded benzene junction (pink square) is taken from reference 1 . R 2 refers to the coefficient of determination, and an R 2 close to 1 indicates nearly perfect data fitting. This suggests that our experiments indeed produce linear oligophenylenes covalently connected to Au electrodes through Au-C bonds.

Supplementary Table 1. Experimental and calculated junction length.
We can see that the measured length difference between the High-G junction (0.1 V) and Low-G junction (1 V) matches very well with the theoretical length difference between single CPP junction and single LPP junction. Notice that, due to the Au electrode snapback distance, the absolute value of the measured length is about ~1 nm shorter than the theoretical results. These support the hypothesis of high bias driven transition from the High-G junction to the Low-G junction.  Figure 3c. These results thus support the proposed transitions from CPP to LPPs. These results demonstrate that the break of C-C bond in [n]CPP does not relate to redox event, thus rules out an electrochemical mechanism.

Modified STM-BJ measurements for [6]CPP
As discussed in the manuscript and shown in Figure 4, the applied high bias can drive the conductance jump from instrumental noise floor to the signature conductance of single LPP junctions. Here, we perform similar modified STM-BJ measurements where we first form the single CPP junction at a low bias and ramp the bias up. As shown in Figure S14, the applied high bias can drive the formation of single LPP junctions.

Supplementary Fig. 13. Modified STM-BJ measurements for [6]CPP. The 2D
conductance-time histograms is compiled from selected traces that show a switch from the High-G to the Low-G under the high bias during the hold period. The conductance profiles are determined from the three regions in the 2D histograms. These results indicate a high bias induced in situ transition from single CPP junctions to single LPP junctions.

First-order kinetic analysis for [6]CPP
We use the simple first-order reaction model to do some rough kinetic analysis 2 : Here we choose t=0 and t=0.015s, and determine k via: We approximate [c]t by the ratio of the number of the Low-G junction to the total number of junction traces. We can then determine k under different biases (see Figure   R8). We see that k is increased from 4.3 s -1 at 0.1 V to 241.3 s -1 at 1 V, implying a ~50fold increase of reaction rate. Supplementary Fig. 14

Total energy calculations of Au-[6]CPP complex under electric field
Using the calculation method mentioned before, we obtain a series of optimized geometries for [6]CPP under different electric field ( Figure S17). We can see that, as the applied electric field increases, the π-complex gradually converts to a more chargeseparated σ-complex. To better understand the electric field driven conversion, we calculate the single point energy of the π-complex ( Figure S17b) and the σ-complex ( Figure S17l) under the electric field strength between 0 V/Å to 0.5 V/Å. We then obtain the field strength dependent energy difference between the π-complex and the σcomplex (Figure 5b). We can see that, as the electric field is increased to > ~0.3 V/Å, the σ-complex becomes more stable than the π-complex as its total energy becomes lower. These results thus rationalize the proposed mechanism of OEEF-driven transition from the π-complex to the σ-complex underlying the EAS process. We can see that as the electric field increases, the π-complex gradually transforms into the charge-separated σ-complex.

Supplementary Notes
Cartesian Coordinates of Optimized Structures.